In radiotherapy and radiosurgery it is of great importance to have reliable information about the correct placement of the patient (specifically, the target to be irradiated with a treatment beam) relative to the beam source. For example, the patient needs to be correctly pre-positioned on the treatment couch so that any finer positioning algorithm will have a good start point for successful execution of the positioning algorithm. Previous approaches to pre-positioning include determination of the patient's position by detecting reflecting markers fixedly attached to the patient's body in a known spatial relationship. Such, however, requires the markers to be kept clean during the positioning procedure and requires a further step to the pre-positioning procedure which involves attaching the markers.
In general, the registration between a video image and a CT dataset is usable for patient motion management in the framework of radiotherapy or radiosurgery and (if accurate enough) also for patient positioning as the video image (such as an infrared video image) is free of ionizing radiation and available in high frame rates. Thus the patient's position can be verified several times per second.
The following two prior art approaches for monitoring the patient's position on the basis of infrared videos are known:
1. Real-time patient monitoring is currently done by fusing point clouds from a range camera with a reference point cloud that either comes from an aligned CT or a range camera's point cloud taken at reference position. Registration between point clouds is instable and computationally demanding. Current Range Cameras are either slow and/or not accurate. Point clouds are often incomplete and noisy which complicates the registration process even more. Range Cameras are expensive, and subject to the surface quality of the object of interest.2. The prior art tries to register two modalities (CT with video) directly. To achieve the minimization they must find a way for one modality to mimic the other modality. In X-Ray/CT Registration e.g. DRRs (digitally rendered radiographs) are a way for the CT modality to ‘mimic’ an X-Ray. Then the registration tries to find the DRR which is most ‘similar’ to the CT. The prior art tries to do the same thing by generating virtual video images from the CT and assumptions about the treatment room lighting and general skin colour, and assuming to know that the patient is naked. All these assumptions are of course inaccurate and error-prone. Even under very good conditions it is doubtful that this virtual image is able to ‘mimic’ the real video image in any way.
The inventors acknowledge the fact that a CT and a thermal image cannot be registered directly as there is no good similarity measure known. It however is possible to register CBCT (cone-beam computed x-ray tomography) or stereo x-ray to the machine isocentre. Using these techniques, a CT can be aligned to the current patient position in an isocentric coordinate system (i.e. a coordinate system which rest relative to/and or in the isocentre). Furthermore, it is possible to register a thermal camera to the isocentric coordinate system using a suitable calibration phantom and a calibration procedure known form the state of the art. So once a CT has been aligned to the live (current) position of the patient in the treatment-room, the CT is aligned in the same coordinate system as the video/thermal camera. Being able to calculate now the position of the camera relative to the CT we can project the CT surface into the camera's image.
Thus we receive a reference set of coloured 2D/3D points of the patient's surface which can serve as the basis for the application of any RGB-D or pure visual registration algorithm (which both may be known from the state of the art) for registering a current image against the reference set and realize patient motion management.
The present invention is designed to achieve patient motion management for pre-positioning or monitoring the patient's position on the basis of a planning CT and infrared images taken after placing the patient ready for therapy.
The present invention can be used in connection with a system for image-guided radiotherapy such as ExacTrac®, a product of Brainlab AG.
Aspects of the present invention, examples and exemplary steps and their embodiments are disclosed in the following. Different exemplary features of the invention can be combined in accordance with the invention wherever technically expedient and feasible.